Roof Ventilation and Vapour Barriers

Where moisture in a pitched roof comes from, why the vapour barrier goes on the warm side and the ventilation gap on the cold side, and how to spot mistakes.

Some homeowners who call a roofer about a leak do not actually have a leak at all. A stain on the ceiling, soaked insulation or darkened timber rafters can come not from rain but from water vapour produced inside the house. When that vapour reaches a cold surface within the roof build-up, it condenses. Two tools keep it under control: the vapour barrier and ventilation beneath the roof covering.

Where the moisture comes from

Indoor air always carries some water vapour — from cooking, showers, drying laundry and even breathing. Warm air rises and tries to pass into the roof through every gap in the ceiling: light fittings, the joint between ceiling and wall, service penetrations. In winter the surfaces just under the roof covering are cold, and the vapour settles on them like dew.

In a single season this may go unnoticed. Over years, however, wet insulation loses much of its ability to hold heat, timber starts to rot and metal fixings corrode. That is why the layers of a pitched roof have to deal not only with rain from outside but with moisture from inside.

The vapour barrier: warm side, no gaps

A vapour barrier is a film or membrane fitted on the room side of the insulation — the warm side. Its job is to limit how much vapour gets into the insulation. What matters most here is not the product itself but the quality of installation.

• Sheets overlap and every joint is sealed with a dedicated tape.
• Edges meeting walls, chimneys and roof windows are sealed airtight.
• Cables and light points are routed without puncturing the barrier; a service void between the barrier and the plasterboard makes this much easier.
• Tears and areas riddled with staples are patched with tape.

Even one open seam can become a point where vapour concentrates, because air always takes the easiest route.

The ventilation gap: airflow on the cold side

On the outer side of the insulation the logic is reversed: the aim is to let moisture escape. Counter-battens create an air gap between the roof covering and the breathable underlay. Air enters this gap low down at the eaves and leaves at the ridge.

For this to work, both ends must be open. The eaves are closed with a vent comb or perforated soffit so birds and insects stay out while air passes. At the ridge a ventilated ridge system or roof vents are installed. Where roof windows, chimneys or valleys interrupt the air path, extra outlets are planned for those areas.

How ventilation is arranged also depends on how the loft is used. In a cold loft, insulation lies on top of the ceiling and the whole loft volume is ventilated through grilles in the gables or eaves and an outlet at the ridge. The loft hatch must then be airtight and insulated, or indoor air will flow straight into the cold zone. In a warm attic, insulation sits between the rafters along the slope and air moves only in the thin gap under the covering. Remember that each rafter bay has its own air channel: a channel interrupted above and below a roof window needs its own inlet and outlet.

Don’t confuse a breathable membrane with a vapour barrier

The two materials can look alike but do opposite jobs. A breathable membrane on the outer side keeps liquid water out while letting vapour through. A vapour barrier holds vapour back. Swap their positions and the insulation becomes a moisture trap sealed on both sides.

The simplest check for a buyer is the technical data sheet, which states the vapour permeability and which way the product should face. The roll itself is usually marked to show which side goes towards the insulation.

Warning signs and a checklist

In an existing house, these signs can point to a moisture imbalance under the roof:

• droplets or frost on the underside of the covering in the loft on winter mornings;
• yellow ceiling stains appearing during dry weather;
• mould in the corners of attic rooms;
• darkened rafters and a musty smell;
• attic rooms overheating badly in summer.

When a new roof is being built, ask these questions at handover of each stage: are the vapour barrier joints taped, is there an air inlet at the eaves, is there an outlet at the ridge, and which face of the membrane points outwards? Once the covering goes on these layers are hidden, so they need to be checked as concealed work.

Zaferoğlu İnşaat’s construction service covers the full sequence from excavation to installing the roof covering. Decide early whether the attic will be heated, because the order of vapour barrier, insulation and ventilated gap follows from that.